1. Field of the Invention
The present invention relates to a process for conversion of natural gas to hydrocarbon products. More specifically, the invention relates to a Fischer-Tropsch synthesis process characterized by including a Fischer-Tropsch tail gas recycle and a carbon dioxide removal, but no carbon dioxide recycle. The invention further relates to a plant for carrying out the process.
2. Description of the Related Art
Steam reforming processes produce synthesis gas (syngas) with a ratio module H2/CO between 2 and 5. By reducing the steam-to-carbon ratio (S/C ratio), the H2/CO ratio can be lowered. Addition of carbon dioxide (CO2) or recycling of CO2 can also be used to reduce the S/C ratio in syngas manufacturing.
The Fischer-Tropsch (FT) process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally with the formula CnH(2n+2). The more useful reactions produce alkanes as follows:(2n+1)H2+n CO→CnH(2n+2)+n H2O  (1)where n is typically 10-20. The formation of methane (n=1) is unwanted. Most of the alkanes produced tend to be straight-chain, suitable as diesel fuel. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons.
The FT process is a key component of gas-to-liquids (GTL) technology, and it can produce synthetic lubrication oils and synthetic fuels, typically from natural gas, coal or biomass.
For GTL plants based on FT synthesis, the required H2/CO ratio typically is approximately 2.0. This is normally obtained by operating within low S/C ratios and by recycling a small part of the excess tail gas from the FT synthesis. If too much FT tail gas is recycled, then the H2/CO ratio will become too low. For FT synthesis based on cobalt catalyst systems, the reactants are H2 and CO, while N2, CO2 and CH4 all are inert in the synthesis. Too high amounts of these inert compounds entail the disadvantage that the conversion to hydrocarbons in the FT section is reduced. The FT tail gas contains the inert components from the FT synthesis (CO2 and N2) and unconverted H2 and CO together with light hydrocarbons formed in the FT synthesis. Therefore a recycling of FT tail gas acts to adjust the syngas module through the presence of CO2, but also to recycle the unconverted hydrocarbons.
The present invention is based on combining the removal of CO2 from the produced syngas with a recycling of FT tail gas. Normally those skilled in the art would only use one of these techniques to obtain the target H2/CO ratio. The removal of CO2 can be carried out by several techniques, including CO2 wash systems or CO2 selective membranes. Such systems are used in syngas plants for ammonia or in CO plants for the final clean-up of the syngas. However, the CO2 removal section is normally regarded as an extra capital investment, and it is not used in GTL plants based on FT synthesis.
The prior art within the field of the present invention distinguishes between (1) FT tail gas recycle but no CO2 removal or recycle and (2) FT tail gas recycle and CO2 removal and recycle. These two scenarios can be described as follows:
(1) FT Tail Gas Recycle but no CO2 Removal or Recycle
Natural gas is mixed with a small amount of hydrogen, compressed (if required) and heated to the required temperature before entering the feed purification section, which consists of one or several catalytic reactors. In said section, impurities such as (but not limited to) sulfur, chlorine and heavy metals are removed from the natural gas. The natural gas is then mixed with steam, whereby the desired S/C ratio is obtained. The mixture of natural gas and steam is then (after a pre-heating, if this is decided beneficial) mixed with an off-gas coming from the downstream FT synthesis process, the so-called FT tail gas. This mixture of natural gas, steam and FT tail gas is converted into a synthesis gas, mainly consisting of hydrogen, carbon monoxide, carbon dioxide and a small amount of residual methane, in the reforming section. The mixing ratio between natural gas and FT tail gas can be used to control the ratio between hydrogen and carbon monoxide in the produced synthesis gas. The reforming section can consist of one or several catalytic reactors and/or reformers, for example an adiabatic pre-reformer, a tubular reformer and/or an autothermal reformer (ATR) consisting of a burner for partial oxidation of the feed stream with oxygen and an adiabatic catalyst bed. The synthesis gas leaving the reforming section is cooled in a waste heat boiler and boiler feed water pre-heaters before being further treated in additional catalytic reactors, if required, for example for final adjustment of product gas composition, or removal of impurities. The synthesis gas is then cooled further, and the excess process water is condensed and removed. The almost dry synthesis gas is then sent to the downstream Fischer-Tropsch synthesis process as make-up gas.
(2) FT Tail Bas Recycle and CO2 Removal and Recycle
Natural gas is mixed with a small amount of hydrogen, compressed (if required) and heated to the required temperature before entering the feed purification section, consisting of one or several catalytic reactors. In the feed purification section, impurities such as (but not limited to) sulfur, chlorine and heavy metals, are removed from the natural gas. The purified natural gas is then mixed with steam, whereby the desired S/C ratio is obtained. The mixture of natural gas and steam is then (after pre-heating if this is decided to be beneficial) mixed with an off-gas coming from the downstream FT synthesis process, the so-called FT tail gas, and CO2 is recycled from the downstream CO2 removal process. This mixture of natural gas, steam, CO2 and FT tail gas is converted into a synthesis gas, mainly consisting of hydrogen, carbon monoxide, carbon dioxide and a small amount of residual methane, in the reforming section. The mixing ratio between natural gas, CO2 and the FT tail gas can be used to control the ratio between hydrogen and carbon monoxide in the produced synthesis gas. The reforming section can consist of one or several catalytic reactors and/or reformers, for example an adiabatic pre-reformer, a tubular reformer and/or an autothermal reformer (ATR) consisting of a burner for partial oxidation of the feed stream with oxygen and an adiabatic catalyst bed. The synthesis gas leaving the reforming section is cooled in a waste heat boiler and boiler feed water pre-heaters before being further treated in additional catalytic reactors, if required, for example for final adjustment of the product gas composition or for removal of impurities. The synthesis gas is then cooled further, and the excess process water is condensed and removed. The almost dry synthesis gas is then sent to the CO2 removal unit, where a larger or smaller part of the CO2 is removed from the synthesis gas. The synthesis gas, now containing less CO2, is sent to the downstream FT synthesis process as make-up gas, and after compression in the CO2 recycle compressor, the CO2-rich stream is recycled, either fully or partly, to the reforming section. In case the CO2 is only partly recycled, the excess CO2 kept for other use, either before or after compression.
From U.S. Pat. No. 6,375,916 B2 it is known to treat a hydrocarbon feed containing higher hydrocarbons by first pre-reforming the feed to remove or reduce the content of higher hydrocarbons and then passing the effluent from the pre-reformer to an ATR. The process is preferably operated at low S/C ratios, since a low S/C ratio lowers the investment expenses for an ATR plant and reduces the energy consumption in operating the plant. However, this patent is silent about any tail gas recycle and also about removal of CO2 from ATR syngas.
EP 0 516 441 A1 describes a process for the conversion of natural gas into higher hydrocarbons. The natural gas is reacted with steam in a reformer to produce a first product stream containing CO, CO2 and H2, which is then passed to a Fischer-Tropsch reactor without separation of CO2, whereby a second product stream including hydrocarbons and CO2 is produced. This second product stream is passed to a product recovery unit, where the desired hydrocarbon products are recovered, leaving a third product stream containing CO2. At least a portion of this third product stream is recycled to the reformer. The EP application does not mention or envisage the possibility of removing all CO2 from the syngas and avoiding any recycling thereof.
US 2013/0065974 A1 discloses a process for synthesizing hydrocarbons, said process more specifically being an enhanced FT process for the synthesis of sulfur-free, clean burning hydrocarbon fuels, examples of which include synthetic diesel and aviation fuel. In the process, naphtha is a mandatory part of the hydrocarbons produced, and at least a portion of said naphtha is recycled to the syngas generator to obtain an enhanced hydrogen-rich stream and thereby enhance the synthesis of hydrocarbons. The process according to the present invention distinguishes itself from the process of the US application in that no naphtha recycle is involved. The process of the US application does include a CO2 removal step, which is however optional, and if any CO2 is removed, at least part of it is re-introduced into the ATR via the pre-treatment unit to enhance the production of synthetic diesel.
It is well-known to produce a synthesis gas by either ATR or SMR (steam methane reforming) followed by subsequent removal of part of the CO2.
The above in combination with recycling a part of or all of the removed CO2 to the inlet of the reformer is also known. This includes adjusting the H2/CO ratio for the subsequent FT synthesis section to the desired value around 2.
Furthermore it is known to produce liquid fuels from syngas by fluid bed Fischer-Tropsch synthesis. A typical plant produces syngas in one or more reformer trains, each consisting of a desulfurizer, a top fired primary (steam) reformer and a secondary (oxygen-fired autothermal) reformer. Part of the natural gas is bypassed around the primary reformer and fed to the secondary reformer together with recycled tail gas. To adjust the CO2 concentration to the desired level (around 6.5%) for entering the FT synthesis unit, part of the exit stream from the secondary reformer is subjected to CO2 removal without recycling. The CO2 thus removed is exported for recovery as a liquid stream. Thus it is known to remove CO2 without recycling, but the feed stream to the FT synthesis unit still contains a certain amount of CO2 because a part of the CO2-containing stream from the reformer bypasses the CO2 removal step.
Various processes for the conversion of hydrocarbon-containing gases, especially natural gas, to hydrocarbon products are known in the art. Thus, as already mentioned, it is known to convert natural gas to syngas by reaction with steam and optionally oxygen. If the feed contains significant amounts of higher hydrocarbons (C2+), pre-reforming can be used to convert these higher hydrocarbons into syngas according to the reactions:CnHm+n H2O→nCO+(n+m/2)H2  (2)3 H2+COCH4+H2O  (3)CO+H2OCO2+H2  (4)
The syngas thus obtained can then be converted to desired hydrocarbon products by using the FT process.
Autothermal reforming (ATR) is a technology commonly used for the production of syngas, where the conversion of a hydrocarbon feedstock, such as natural gas, is carried out in a single reactor through the combination of partial combustion and adiabatic steam reforming. The ATR reactor consists of a burner, a combustion chamber and a fixed bed catalyst section contained in a refractory lined pressure shell. The key elements in the ATR reactor are the burner and the catalyst bed. Combustion of the hydrocarbon feed is carried out with sub-stoichiometric amounts of air, enriched air or oxygen by flame reactions in a burner combustion zone. The burner provides mixing of the feed streams, and the natural gas is converted in a turbulent diffusion flame, often simplified by the reactionCH4+3/2 O2→CO+2 H2O  (5)
The catalyst bed equilibrates the methane steam reforming reaction (the reverse of reaction (3) above) and the shift reaction (reaction (4) above).
When natural gas, typically a mixture of predominantly methane with some higher hydrocarbons, nitrogen and CO2, is used as the only carbon-containing material in the feed to the reformer, the syngas obtained is not optimal for use in the FT reaction because of the H2/CO ratio. Therefore it is normal practice to remove the CO2, which is co-produced during the reforming process, and recycle the desired quantity back to the reforming section. The addition of this CO2 to the feed changes the H2/CO ratio. Careful control of the amount of recycled CO2 allows a desirable H2/CO ratio to be achieved.